fix(storage): enforce mutual exclusion between Close() and Finalize() in async writers

[kalragauri]

This PR addresses feedback regarding the interaction between Close() and Finalize() in AsyncWriterConnection implementations (#16163 (comment)).

Previously, if Finalize() was called followed by Close(), Close() would still append its payload, but the WriteLoop would prioritize FinalizeStep over CloseStep. The Close() future would hang indefinitely because the closed_ promise was never satisfied. This change enforces mutual exclusion between these two terminal operations, ensuring they fail fast.

[gemini-code-assist]

Code Review

This pull request introduces validation checks in Finalize() and Close() for both buffered and resumed async writer connections to prevent invalid state transitions (such as calling Close() after Finalize(), or calling them multiple times), along with corresponding unit tests. The review feedback highlights critical race conditions in these checks because the lock is released before the futures are moved out, allowing concurrent calls to bypass the .valid() checks. It is recommended to use the existing finalize_ and close_ boolean flags instead, which are safely updated under the lock.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (f1a8e57) to head (5a42300).

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16211    +/-   ##
========================================
  Coverage   92.23%   92.24%            
========================================
  Files        2265     2265            
  Lines      210205   210330   +125     
========================================
+ Hits       193888   194021   +133     
+ Misses      16317    16309     -8     

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